Technical Field
The present invention relates to a low-alloy steel pipe for an oil well and, more particularly, to a high-strength low-alloy steel pipe for an oil well.
Description of the Background Art
A steel pipe for an oil well may be used as a casing or tubing for an oil well or gas well. Both an oil well and a gas well will be hereinafter referred to as “oil well”. As deeper and deeper oil wells are developed, a steel pipe for an oil well is required to have higher strength. Traditionally, steel pipes for oil wells in the 80 ksi strength grade (i.e. with a yield strength in the range of 80 to 95 ksi, i.e. a yield strength in the range of 551 to 654 MPa) or in the 95 ksi grade (i.e. with a yield strength in the range of 95 to 110 ksi, i.e. a yield strength of 654 to 758 MPa) have been employed. Recently, however, steel pipes for oil wells in the 110 ksi strength grade (i.e. with a yield strength in the range of 110 to 125 ksi, i.e. a yield strength in the range of 758 to 861 MPa) are used in more and more cases.
Many deep oil wells that have been recently developed contain hydrogen sulfide, which is corrosive. In such an environment, an increased strength of steel means increased susceptibility of the steel to sulfide stress cracking (hereinafter referred to SSC). Many steel pipes for oil wells that are used in an environment containing hydrogen sulfide are low-alloy steel pipes, because martensitic stainless steel, which has good carbon dioxide gas corrosion resistance, has high susceptibility to SSC.
Although low-alloy steel has a relatively good SSC resistance, such a steel with increased strength has higher susceptibility to SSC. Thus, one needs to come up with various ideas for material designing for a steel pipe for an oil well that are used in an environment containing hydrogen sulfide to increase the strength of the steel pipe and, at the same time, ensure a certain SSC resistance.
To improve the SSC resistance of a steel, WO 2007/007678 discloses (1) improve the cleanliness of the steel; (2) quenching the steel and then tempering it at a high temperature; (3) making the crystal grains (prior austenite grains) of the steel finer; (4) making the particles of carbide produced in the steel finer or more spherical; and other approaches.
The low-alloy steel for an oil well described in this document has a chemical composition that satisfies 12V+1-Mo≥0, and, if it contains Cr, further satisfies Mo—(Cr+Mn)≥0. According to this document, this low-alloy steel for an oil well has a high yield strength that is not lower than 861 MPa and exhibits good SSC resistance even in a corrosive environment with 1 atm H2S.
JP 2000-178682 A discloses a steel for an oil well made of a low-alloy steel containing C: 0.2 to 0.35%, Cr: 0.2 to 0.7%, Mo: 0.1 to 0.5%, and V: 0.1 to 0.3%, where the total amount of precipitated carbide is in the range of 2 to 5 wt. %, of which MC-based carbide accounts for 8 to 40 wt. %. According to this document, this steel for an oil well has good SSC resistance and a yield strength of 110 ksi or higher. More specifically, this document describes that, in constant load tests complying with the TM0177 method A from the National Association of Corrosion Engineers (NACE) (in an aqueous solution of 5% NaCl and 0.5% acetic acid saturated with H2S at 25° C.), this steel for an oil well does not break under a load stress of 85% of its yield strength.
JP 2006-265657 A discloses a method of manufacturing a seamless steel pipe for an oil well, where a seamless steel pipe with a chemical composition having C: 0.30 to 0.60%, Cr+Mo: 1.5 to 3.0% (Mo being not less than 0.5%), V: 0.05 to 0.3% and other components is produced and, immediately after completion of rolling, water-cooled to a temperature range of 400 to 600° C. and, without an interruption, a bainitic isothermal transformation heat treatment is performed in a temperature range of 400 to 600° C. This document describes that this seamless steel pipe for an oil well has a yield strength of 110 ksi or higher, and, in constant load tests complying with the TM0177 method A from NACE, does not break under a load stress of 90% of its yield strength.
WO 2010/150915 discloses a method of manufacturing a seamless steel pipe for an oil well, wherein a seamless steel pipe containing C: 0.15 to 0.50%, Cr: 0.1 to 1.7%, Mo: 0.40 to 1.1% and other components is quenched under a condition that produces prior austenite grains with a grain size number of 8.5 or higher, and tempered in a temperature range of 665 to 740° C. According to this document, this method produces a seamless steel pipe for an oil well in the 110 ksi grade with good SSC resistance. More specifically, this document describes that, in constant load tests complying with the TM0177 method A from NACE, this seamless steel pipe for an oil well does not break under a load stress of at least 85% of its yield strength.
WO 2008/123425 describes a low-alloy steel for oil well pipes with good HIC resistance and SSC resistance in a high-pressure hydrogen sulfide environment and having a yield strength of 758 MPa or more, which contains C: 0.10 to 0.60%, Cr: 3.0% or less, Mo: 3.0% or less and other components, and satisfies the relationship represented by Cr+3Mo≥2.7%, where not more than 10 non-metallic inclusions with a length of their major axis of 10 μm are present in an area of 1 mm2 of an observed cross-section.
Japanese Patent No. 5387799 describes a method of manufacturing a high-strength steel with good sulfide stress cracking resistance, including, after a steel having a predetermined chemical composition is hot-worked, [1] the step of heating the steel to a temperature above Ac1 point and below Ac8 point and then cooling it, [2] the step of reheating the steel to a temperature that is not lower than Ac3 point and rapidly cooling it for quenching, and [3] the step of tempering the steel at a temperature that is not higher than Ac1 point, the steps being performed in this order.
JP 2010-532821 A describes a steel composition containing C: 0.2 to 0.3%, Cr: 0.4 to 1.5%, Mo: 0.1 to 1%, W: 0.1 to 1.5% and other components, where Mo/10+Cr/12+W/25+Nb/3+25×B is in the range of 0.05 to 0.39% and the yield strength is in the range of 120 to 140 ksi.
Japanese Patent No. 5522322 describes a steel for a pipe for an oil well containing C: higher than 0.35% to 1.00%, Cr: 0 to 2.0%, Mo: higher than 1.0% to 10% and other components, where the yield strength is 758 MPa.